Silicon microphones, or silicon based MEMS microphones, also known as acoustic transducers, have been in research and development for many years. The silicon microphones may be widely used in many applications, such as cell phones, tablet PCs, cameras, hearing aids, smart toys and surveillance devices due to their potential advantages in miniaturization, performances, reliability, environmental endurance, costs and mass production capability.
In general, a silicon microphone consists of a fixed perforated backplate and a highly compliant diaphragm with an air gap formed in between. The perforated backplate and the compliant diaphragm, forming a variable air-gap condenser, are typically formed on a single silicon substrate, with one of which being directly exposed to the outside through a back hole formed in the silicon substrate.
Patent application No. WO 02/15636 discloses an acoustic transducer, which has a substrate formed with a back hole therein, a diaphragm made of low stress polysilicon and directly positioned above the back hole of the substrate, and a cover member (equivalent to the said backplate) disposed above the diagram. The diaphragm can be laterally movable within its own plane parallel to the planar surface of the cover member, and thus can release its intrinsic stress, resulting very consistent mechanical compliance.
Patent document PCT/DE97/02740 discloses a miniaturized microphone, in which an SOI substrate is used for formation of the microphone and related CMOS circuits. Specifically, the silicon layer of the SOI substrate is used to form the backplate of the microphone which is directly above a back hole formed in the SOI substrate, while a subsequently deposited polysilicon thin film, serving to be the diaphragm of the microphone, is located above the backplate with an air gap in between and exposed to the outside through both the openings in the perforated backplate and the back hole in the SOI substrate.
FIG. 1 shows a cross-sectional view of an exemplary structure of a conventional silicon microphone. As shown in FIG. 1, the conventional silicon microphone 10 comprises a silicon substrate 100 provided with a back hole 140 therein, a conductive and compliant diaphragm 200 stacked on the silicon substrate 100 with an oxide layer 120 sandwiched in between and serving as an electrode as well as a vibration membrane, a perforated backplate 400 located above the diaphragm 200 and formed with CMOS passivation layers with a metal layer 400b imbedded therein which serves as an electrode plate of the backplate 400, and an air gap 150 provided between the diaphragm 200 and the backplate 400 and with a spacer 300 forming the boundary thereof. The diaphragm 200 and electrode plate of the backplate 400 forms a variable condenser, which has an extraction electrode 410 for the diaphragm 200 and an extraction electrode 420 for the backplate 400. More details of the conventional microphone 10 are described in the international application No. PCT/CN2010/075514 and omitted herein for sake of conciseness.
As can be seen from the structure of the above described silicon microphone 10, the diaphragm 200 is defined by and exposed to the outside through the back hole 140 formed in the substrate 100 by a wet etching or dry etching process. Since the silicon substrate 100 has a typical thickness of, for example, 400 μm or more, and the achievable precision of the back hole wet etching or dry etching of the substrate 100 is typically at ±20 μm, there exists a problem with the conventional silicon microphone 10 that the imprecise back hole etching process would cause a variation in the vibrating area of the diaphragm 200 (i.e. the area shown between two vertical dash lines in FIG. 1), and thus cause a variation in the sensitivity of the silicon microphone 10, which renders the silicon microphone 10 a low repeatability and reproducibility in performance.